As is well-known, in the electrical engineering sector, the term “rotor” is used to define the drive shaft of an electric motor.
The rotor, in a machine with moving parts, is the set of rotating parts situated opposite the stator which is the stationary part.
Eddy currents are induced on the ferromagnetic rotor of an electric machine which is subject to a variable magnetic field; more specifically superficial circulation currents which are proportional to the dimensions of the rotor and to the square of the frequency—and, therefore, to the speed of rotation of the rotor—are induced.
In a permanent-magnet motor the eddy currents which circulate in the rotor made of ferromagnetic materials result in heating (also locally) of the magnetic material which forms the poles since the magnets constitute an electrically conductive continuum.
Usually the heat present in the rotor is removed owing to a (natural or forced circulation) fluid which flows over the moving parts, ensuring that the temperature of the rotor remains within a certain limit. With an increase in the speed the superficial currents which affect the magnet are considerable and result in significant energy losses and an increase in the temperature of the magnet, considering also that with an increase in the speed of rotation cooling of the rotor becomes gradually more problematic.
In this situation it is possible that the temperature is reached such that the magnetic induction flux tends towards zero, preventing the correct operation of the electric machine; in other words, the electric machine starts to lose efficiency until it stops.
If the temperature rises beyond what is known as the “Curie temperature”, the overheating may result in loss of the properties which are typical of magnetic materials which consequently become paramagnetic; in other words, they permanently lose their magnetic properties.
It is possible to reduce the circulating currents (and therefore the dissipation losses) using magnets which are laminated or insulated with respect to each other.
Unfortunately, this solution is not without constructional difficulties.
In fact the rotor produced with insulated laminated magnets is not homogeneous from the point of view of the mass density.
In high-speed applications, i.e. at speeds typically of between 10,000 rpm and 300,000 rpm, this results in non-optimum operation with high losses and low yield, making use of this solution impractical.
The technical problem is to provide a homogeneous magnetic rotor which maintains its homogeneous characteristics also at high speeds, reducing at the same time the eddy currents, so as to overcome the problems of the prior art.